The present invention generally relates to an apparatus for producing an Auger image and, in particular, relates to such an apparatus which is substantially independent of the topographical contour of the sample under test.
Auger analysis in a sophisticated analytical technique whereby the surface of a sample is examined for its elemental composition. When discussing Auger systems, the term "surface" is generally considered to be that portion of the sample which is on the order of a few atomic layers deep. During an Auger analysis, a primary electron source bombards a segment of the sample surface to release secondary electrons (i.e. Auger electrons) therefrom, which secondary electrons are collected and analyzed. The liberated particles are usually analyzed as a function of their energy.
As well known in the art, one particularly convenient means for evaluating the data generated by an Auger system is to create an Auger image, or map. To create such a map, the technologist generally measures the intensity of an Auger peak for a two-dimensional array of points on the sample being tested. In such an analysis, the primary electron beam is digitally controlled by a computer and stepped through a rectangular area of the sample surface. The points on the map are usually scanned in a raster pattern simlilar to that used for producing a television image. To form such an Auger image of a surface, the magnitude of the Auger peaks are first measured at each point in the raster matrix. This information is generally stored in a memory device and later displayed on, for example, an oscilloscope or other form of recorder where the intensity at each point of the raster is proportional to the magnitude of the Auger peak.
The intensity of a particular Auger peak is generally obtained by measuring the magnitude, N(e.sub.p) of the peak at the energy, e.sub.p, giving maximum intensity and subtracting the background magnitude N(e.sub.b) at an energy e.sub.b sufficiently removed from the peak that Auger electrons forming the peak do not contribute. In conventional computer-controlled Auger systems, Auger maps are determined by the following steps: First, an electron-pass energy, e.sub.p, is selected. After the pass energy is determined and the mechanism set, the number of incidences N(e.sub.p) is measured and stored for each point in a selected line of the raster matrix. Thereafter, the pass energy for the detector is set at another base line, e.sub.b, representative of the electron energy of the background which is present. Thereafter, the incidences of background N(e.sub.b) is measured for each point in the same matrix. These measurements N(e.sub.p) and N(e.sub.b), taken at e.sub.p and e.sub.b, respectively, are repeated for each line in the matrix. From the accumulated data an Auger image is constructed by conventional arithmetic processing equipment by subtracting the number of counts at e.sub.b from the number of counts e.sub.p at each point in the matrix.
A first order topographical correction can be achieved by dividing the peak height determined from above by the background incidences, i.e., [N(e.sub.p)-N(e.sub.b)]/N(e.sub.b). This is a fairly accurate correction to the topographical variations which modulate the background and peak height uniformly. This particular method is advantageous in that it is independent of the incident beam current since both the background and the peak are proportional to the excitation beam. Therefore, beam current variations having a period longer than the time required to scan each line do not affect the normalized Auger intensities.
Unfortunately, the above normalization scheme only removes beam current noise of rather low frequency. For example, if a particular line of the matrix contains 200 measurement points with a typical measurement time at each point of ten milliseconds, the elapsed time between the peak and the background measurements is therefore two seconds. Hence the technique is only effective in removing noise components having a frequency less than 0.5 Hz.
Another disadvantage is that by the use of equal measurement intervals at each spatial point, non-uniform statistical noise levels are created when topographical effects, i.e., surface depth variations, vary the signal magnitude either through scattering, absorption or miscellaneous reflections. Hence, even using a first order topographical correction, the variation of the Auger peak due to actual compositional changes of the surface is difficult to distinguish from changes due to noise variations along each particular line, as well as between different lines. As a result, topological variations can result in a complete mischaracterization of the elemental composition of a surface.